1. Field of the Invention
During the past three decades it has been observed that camptothecin (CPT) and most of the highly lipophilic derivatives of camptothecin (HLCD) in their lactone form are poorly water soluble. For example, less than 5 micrograms of drug will dissolve in one milliliter of water to form a solution at a pH of 2 to 6. A range of pH from 2 to 6 maintains the dissolved camptothecin in the lactone form. Camptothecin and many of its poorly water soluble derivatives are known potent anticancer drugs, however, their very poor water solubility has prevented their use in the treatment of human cancer. The potency of these anticancer drugs was determined by their ability to inhibit in vitro and in vivo tumor cell growth. This invention solves the poor solubility problems of camptothecins and its derivatives. Thus, the purpose of this invention is to overcome the poor solubility of highly lipophilic camptothecin derivatives in their lactone form by designing novel formulations of the drug (at sufficient concentrations) which can be administered orally, topically or parenterally for the purpose of treating human patients with cancer.
2. Description of the Related Art
Introduction
A. DNA Topoisomerases
Several clinically important anticancer drugs kill tumor cells by affecting DNA topoisomerases. Topoisomerases are essential nuclear enzymes that function in DNA replication and tertiary structural modifications, such as overwinding, underwinding, and catenation, which normally arise during cellular replication, transcription, and perhaps other DNA processes. Two major topoisomerases have been identified, both of which are ubiquitous to eukaryotic cells: (1) Topoisomerase I (topo I) cleaves single stranded DNA; and (2) Topoisomerase II (topo II) cleaves double stranded DNA. Topoisomerase I is involved in DNA replication; it relieves the torsional strain introduced ahead of the moving replication fork.
Topoisomerase I, purified from human colon carcinoma cells or calf thymus, has been shown to be inhibited by camptothecin and many of its derivatives. Camptothecin, and water soluble camptothecin derivatives including CPT-11, topotecan, 9-amino camptothecin, 9-nitro camptothecin, DX8951 and 7-(4-methylpiperazinomethylene)-10,11-methylenedioxy camptothecin, 10,11-methylenedioxy camptothecin and 10,11-ethylenedioxy camptothecin have either been studied preclinically or used in clinical trials to treat certain types of human cancer. To date, there have been no clinical studies in human patients involving poorly water soluble highly lipophilic camptothecins, other than for camptothecin (in the late 1970's).
This absence of clinical use of lipophilic camptothecins has been due to the lack of pharmaceutical formulations which allow the direct administration of the poorly water soluble camptothecin lactone species to human patients with cancer. For the purpose of this invention, examples of highly lipophilic camptothecin derivatives include camptothecin, 10-hydroxy-7-ethyl camptothecin (SN38), 7-ethyl camptothecin (SN22), 10,11-methylenedioxy camptothecin, 10,11-ethylenedioxy camptothecin and other poorly water soluble derivatives of camptothecin which are active antitumor agents.
For the purpose of this invention, poorly water soluble and highly lipophilic camptothecin derivatives (referred to as "HLCD" for the purposes of this invention) are defined interchangeably as any A- and/or B-ring substituted camptothecin which have a water solubility of less than 5 micrograms per milliliter of water. Also for the purposes of the instant invention, the terms "highly lipophilic" and "poorly water soluble" are used interchangeably to describe their fundamental bioavailability and chemical behavior. Poorly water soluble camptothecin derivatives use the same mechanism to inhibit Topo I; they stabilize the covalent complex of enzyme and strand-cleaved DNA, which is an intermediate in the catalytic mechanism. These compounds have no binding affinity for topoisomerase I but do bind with measurable affinity to the enzyme-DNA complex. The stabilization of the topoisomerase I "cleavable complex" by camptothecin and its derivatives is readily reversible.
Although camptothecin and the aforementioned poorly water soluble camptothecin derivatives have no effect on topoisomerase II, these camptothecin derivatives stabilize the Topoisomerase I--DNA "cleavable complex" in a manner analogous to the way in which epipodophyllotoxin glycosides and various anthracyclines inhibit topoisomerase II.
Inhibition of topoisomerase I by camptothecin and highly lipophilic camptothecin derivatives induces protein-associated DNA single-strand breaks. Virtually all of the DNA strand breaks observed in vitro cells treated with camptothecin are protein linked. However, an increase in unexplained protein-free breaks can be detected in L1210 cells treated with camptothecin. The compounds appear to produce identical DNA cleavage patterns in end-labeled linear DNA. It has not been demonstrated that camptothecin or highly lipophilic camptothecin derivatives cleaves DNA in the absence of the topoisomerase I enzyme.
B. Activity of Highly Lipophilic Camptothecin Derivatives is Cell Cycle Specific
The activity of highly lipophilic camptothecin derivatives is cell cycle specific. The greatest quantitative biochemical effect observed in cells exposed to camptothecin and its derivatives is DNA single-strand breaks that occur during the S-phase. Because the S-phase is a relatively short phase of the cell cycle, longer or repetitive exposures to the drugs results in increased cell killing. Brief exposure of tumor cells to the drugs produces little or no cell killing, and quiescent cells are refractory. These results are likely due to two factors:
(1) The drugs inhibit topoisomerase I reversibly. Although they may produce potentially lethal modifications of the DNA structure during DNA replication, the breaks may be repaired after washout of the drug; and PA1 (2) Cells treated with topo I inhibitors, such as camptothecins tend to stay in G0 of the cell cycle until the drug is removed and the cleaved DNA is repaired. Inhibitors of these enzymes can affect many aspects of cell metabolism including replication, transcription, recombination, and chromosomal segregation. PA1 wherein R.sub.1 and R.sub.2 together may represent --X.sub.1 --X.sub.2 --X.sub.3 -- and wherein X.sub.1, X.sub.2, X.sub.3 may be CR.sub.5 R.sub.6, O, S or NR.sub.7 ; and wherein R.sub.5, R.sub.6, R.sub.7 =H, lower alkyl, alkoxy, acyloxy, hydroxy, acyl, halo, amido, or cyano group; PA1 wherein R.sub.2 and R.sub.3 together may represent --X.sub.1 --X.sub.2 --X.sub.3 -- and wherein X.sub.1, X.sub.2, X.sub.3 may be CR.sub.5 R.sub.6, O, S or NR.sub.7 ; and wherein R.sub.5, R.sub.6, R.sub.7 =H, lower alkyl, alkoxy, acyloxy, hydroxy, acyl, halo, amido, or cyano group; PA1 wherein R.sub.3 and R.sub.4 together may represent --X.sub.1 --X.sub.2 --X.sub.3 -- and wherein X.sub.1, X.sub.2, X.sub.3 may be CR.sub.5 R.sub.6, O, S or NR.sub.7 ; and wherein R.sub.5, R.sub.6, R.sub.7 =H, lower alkyl, alkoxy, acyloxy, hydroxy, acyl, halo, amido, cyano group; PA1 wherein R.sub.1 and R.sub.2 together may represent --X.sub.1 --X.sub.2 --X.sub.3 -- and wherein X.sub.1, X.sub.2, X.sub.3 may be CR.sub.5 R.sub.6, O, S or NR.sub.7 and wherein R.sub.5, R.sub.6, or R.sub.7 =H, lower alkyl, alkoxy, acyloxy, hydroxy, acyl, halo, amido, or cyano group; PA1 wherein R.sub.2 and R.sub.3 together may represent --X.sub.1 --X.sub.2 --X.sub.3 -- and wherein X.sub.1, X.sub.2, X.sub.3 may be CR.sub.5 R.sub.6, O, or NR.sub.7 and wherein R.sub.5, R.sub.6, or R.sub.7 =H, lower alkyl, alkoxy, acyloxy, hydroxy, acyl, halo, amido, or cyano group; and PA1 wherein R.sub.3 and R.sub.4 together may represent --X.sub.1 --X.sub.2 --X.sub.3 -- and wherein X.sub.1, X.sub.2, X.sub.3 may be CR.sub.5 R.sub.6, O, S or NR.sub.7 and wherein R.sub.5, R.sub.6, or R.sub.7 =H, lower alkyl, alkoxy, acyloxy, hydroxy, acyl, halo, amido, or cyano group. PA1 (1) direct administration of HLCD allows the clinician to tailor the administration of the active cytoxic species (lactone stable form of HLCD) to suit the patient's tolerance; PA1 (2) direct administration of HLCD overcomes interpatient variability which may be due to polymorphism of key enzyme(s) in the metabolism of CPT-11 to 7-ethyl-10-hydroxy camptothecin; PA1 (3) clinicians can more consistently optimize the drug dosage and schedule to achieve the maximum tolerated dose of HLCD which is likely to lead to the most beneficial clinical anti-cancer effect; and PA1 (4) direct administration of an HLCD in the lactone form will have a generally superior ability to penetrate tissue than the direct administration of water soluble camptothecin derivatives in the lactone stable or carboxylate forms. PA1 (1) solutions and suspensions comprising lactone stable HLCD; PA1 (2) formulations of lactone stable HLCD suitable for parenteral administration; PA1 (3) oral formulations of lactone stable HLCD; and PA1 (4) use of formulations of HLCD for the treatment of localized complications of cancer by direct administration via instillation into various body cavities. PA1 wherein R.sub.1 and R.sub.2 together may represent --X.sub.1 --X.sub.2 --X.sub.3 -- and wherein X.sub.1, X.sub.2, X.sub.3 may be CR.sub.5 R.sub.6, O, S or NR.sub.7 and wherein R.sub.5, R.sub.6, R.sub.7 =H, lower alkyl, alkoxy, acyloxy, hydroxy, acyl, halo, amido, or cyano group; PA1 wherein R.sub.2 and R.sub.3 together may represent --X.sub.1 --X.sub.2 --X.sub.3 -- and wherein X.sub.1, X.sub.2, X.sub.3 may be CR.sub.5 R.sub.6, O, S or NR.sub.7 and wherein R.sub.5, R.sub.6, R.sub.7 =H, lower alkyl, alkoxy, acyloxy, hydroxy, acyl, halo, amido, or cyano group; PA1 wherein R.sub.3 and R.sub.4 together may represent --X.sub.1 --X.sub.2 --X.sub.3 -- and wherein X.sub.1, X.sub.2, X.sub.3 may be CR.sub.5 R.sub.6, O, S or NR.sub.7 and wherein R.sub.5, R.sub.6, R.sub.7 =H, lower alkyl, alkoxy, acyloxy, hydroxy, acyl, halo, amido, or cyano group;
C. Lactone Form of Highly Lipophilic Camptothecin Derivatives Increases Antitumor Activity and Reduces Water Solubility
Utilizing HPLC and NMR techniques, researchers have demonstrated that camptothecin and many of it's derivatives undergo an alkaline, pH-dependent hydrolysis of the E-ring lactone. The slow reaction kinetics allows one to assess whether both the lactone and non-lactone forms of the drug stabilizes the topoisomerase I-cleaved DNA complex. Studies indicate that only the closed lactone form of the drug helps stabilize the cleavable complex. This observation provides reasoning for the high degree of drug activity observed in solid tumor models. Tumor cells, particularly hypoxic cells prevalent in solid neoplasms, have relatively lower intracellular pH levels than normal cells. At pH levels below 7.0, the lactone E-ring form of camptothecins predominates. Thus, the inventors believe that camptothecins will be more effective at inhibiting topoisomerase I in an acidic environment than in cells having higher intracellular pH levels.
One of the objects of this invention is to provide lactone stable poorly water soluble camptothecin derivatives as the basis of the claimed subject matter. For this invention, lactone stable camptothecin derivatives are defined as poorly water soluble A- and/or B-ring substituted camptothecins which are dissolved in N-methyl-2-pyrrolidinone (referred to as "NMP") in the presence of an acid with or without additional excipients as desired. The inventors have discovered that highly lipophilic camptothecins display an unusually high degree of solubility (greater than 1.0 mg per milliliter)in N-methyl-2-pyrrolidinone (referred to as "NMP"). NMP, as a pharmaceutical excipient, is safe for human administration and has been found by the inventors to be chemically inert with respect to poorly water soluble camptothecins. The presence of an acid in the solution further stabilizes the lactone E-ring form of the HLCD; this is particularly useful when additional excipients are used and when the drug formulation is diluted with aqueous media. For the purpose of this invention, lactone stable camptothecin and highly lipophilic camptothecin are used interchangeably.
D. Camptothecin and Highly Lipophilic Camptothecins
In 1966, Wall and Wani isolated camptothecin from the plant, Camptotheca acuminata. In the early 1970's camptothecin reached Phase I and Phase II human trials and was found to have antitumor activity, but it caused unpredictable myelosuppression and hemorrhagic cystitis. It is important to note that all of these studies used sodium hydroxide formulations of camptothecin which greatly increased the water solubility of the molecule due to base mediated hydrolysis of the lactone E-ring to form the carboxylate species of camptothecin in appreciable quantities. At that time, however, it was not recognized that the lactone E-ring species of camptothecin had significantly (e.g., greater than 10 fold) greater anti-tumor activity than the carboxylate form of camptothecin. Phase II studies with sodium camptothecin were limited because patients given sodium camptothecin experienced unpredictable and severe myelosuppression, gastrointestinal toxicity, hemorrhagic cystitis, and alopecia. Clinical trials with sodium camptothecin (referred to as "SCPT" for the purposes of this invention) were eventually discontinued because of these unpredictable toxicities and the lack of consistent antitumor activity.
To demonstrate the utility and novelty of the present invention, it is useful to review the literature on human clinical trials conducted with SCPT administered parenterally to human patients with cancer. Gottlieb and coworkers (Cancer Chemotherapy Reports 54:461; 1970) reported on clinical studies with the sodium salt of camptothecin (SCPT) which were begun at the Baltimore Cancer Research Center in January 1969. In this clinical trial, SCPT was administered as a rapidly running i.v. solution over a 5-10 minute period at a concentration of 2 mg of SCPT per milliliter of saline. Doses of SCPT ranged from 0.5 to 10.0 mg/kg of actual or ideal body weight (whichever was less). These investigators reported that because hemorrhagic cystitis was noted in several of the early trials, patients receiving camptothecin sodium were well hydrated either i.v. or orally for 72 hours after drug administration. It is interesting to note that the mean urine recovery of camptothecin was 17.4% over the first 48 hours (range: 3.6-38.9%) with most of the drug excretion occurring in the initial 12 hours. When these investigators excluded the 5 patients with impaired urinary excretion, the mean urine recovery of camptothecin was 22.8%. These investigators noted that unmetabolized camptothecin in high concentrations rapidly appeared in the urine after i.v. drug administration and went further to state that this finding probably accounted for the sterile hemorrhagic cystitis noted in 3 moderately dehydrated patients. Although maintaining a copious urine outflow seemed to prevent this complication, the investigators explored various alterations in urine pH as another possible way of decreasing the risk of this debilitating type of toxicity.
Muggia et. al. (Cancer Chemotherapy Reports 56:515; 1972) reported results of a Phase I clinical trial in fifteen patients treated with SCPT at four weekly dose levels ranging from 20-67 mg/m.sup.2. No clinical benefit was observed in eight patients with measurable disease who were treated with the 5-day courses at dose levels associated with toxicity. The drug was administered in concentrations of 1 to 10 mg/ml and it was always administered by intravenous push. Cystitis was the most prominent non-hematologic toxic effect observed in this study. Bladder toxicity was dose limiting in three patients receiving doses of 20 to 30 mg/m.sup.2, and occurred in two additional patients at doses of 44 and 30 mg/m.sup.2. Cystitis, another toxic effect occurring frequently after treatment with SCPT, was not predicted by preclinical toxicologic studies. Their clinical experience suggested that the occurrence of cystitis may be related to the duration of the patient's exposure to the drug when administered as the carboxylate form. Camptothecin is excreted unchanged by the kidneys, although a large percentage of the drug administered cannot be accounted for in the urine and is likely conjugated in the liver to form the glucuronide and excreted via the hepatobiliary route. It is possible that relatively less drug is excreted in the urine of animals since an extremely active transport of camptothecin into bile has been demonstrated. Alternatively, these investigators postulated that the mucosa of the human bladder is more susceptible to the toxic action of camptothecin or that the effect on the human bladder is due to some unrecognized camptothecin metabolite.
Moertel and coworkers reported results of a Phase II Study of Camptothecin (NSC-100880) in the Treatment of Advanced Gastrointestinal Cancer (Cancer Chemotherapy Reports 56:95; 1972.) These investigators administered camptothecin sodium dissolved in physiologic saline at a concentration of 2 mg/ml and administered by rapid intravenous infusion over 5-10 minutes. Two schedules of administration were used in this study: (a) a single injection repeated at 3-week intervals; and (b) a 5-day course repeated ever 4 weeks. The initial dose for the single-dose method was 180 mg/m.sup.2. Because of toxic effects which were considered excessive by the investigators, later patients were treated at doses ranging between 90 and 120 mg/m.sup.2. Dosages for the 5-day course ranged between 11 and 22 mg/m.sup.2 /day (total course, 55-110 mg/m.sup.2). Diarrhea was only a problem at higher doses, but then could be quite severe to the point of fecal incontinence and persistent for as long as 4 weeks. Cystitis usually began about 7-10 days after treatment and was characterized clinically by dysuria and frequency. With more severe toxicity, gross hematuria developed. Pathologically, this was characterized by multiple necrotic ulcerations which could involve the entire urinary tract from kidney pelvis to bladder. According to these investigators, the occurrence of hemorrhagic cystitis did not preclude further treatment with camptothecin, and its severity could be titrated down by lowering the dose in subsequent courses. These investigators also reported that the more prolonged schedule produced more severe toxicity at a given total dose level, but the difference was not as great as might have been predicted by preclinical animal studies. These investigators proposed that a reasonable initial dose of SCPT is 110-120 mg/m.sup.2 for the single-injection method or 17 mg/m.sup.2 /day (total dose, 85 mg/m.sup.2) for the 5-day course. They noted that after 2 months (8 or 9 weeks) only two of their 61 patients showed evidence of partial objective improvement, and none showed improvement at 3 months. Both patients who demonstrated an objective response at 2 months had large bowel cancer. These investigators concluded that camptothecin "is a drug of protean and unpredictable toxicity that has no clinical value in the management of gastrointestinal cancer." See Tables 1, 2, 3, and 4.
TABLE 1 ______________________________________ Toxic Reactions: Single-Dose Administration of Sodium Camptothecin (Moertel et. al. Cancer Chemotherapy Reports 56:95; 1972.) Nonhematologic Toxicity No. of patients with: No. of Dose patients (mg/m.sup.2) treated Diarrhea Cystitis ______________________________________ 90 10 -- 1 100 6 -- 2 110 2 1 1 120 7 4 2 180 9 2 3 ______________________________________
TABLE 2 ______________________________________ Toxic Reactions: 5 Consecutive Day Administration of Sodium Camptothecin (Moertel et. al. Cancer Chemotherapy Reports 56:95; 1972.) Nonhematologic No. of patients with: No. of Dose patients (mg/m.sup.2 .times. 5) treated Diarrhea Cystitis ______________________________________ 11 2 -- 1 15 9 1 4 17 5 4 2 20 10 4 6 22 1 1 -- ______________________________________
TABLE 3 ______________________________________ Relationship of Method of Administration to Cystitis (Moertel et. al. Cancer Chemotherapy Reports 56:95; 1972.) Method of administration Single dose 5-Day course Cystitis (% of 34 patients) (% of 27 patients) ______________________________________ 24 48 (P &lt; 0.05) ______________________________________
TABLE 4 ______________________________________ Objective Responses (Moertel et. al. Cancer Chemotherapy Reports 56:95; 1972.) ______________________________________ Single-dose method (34 patients) Time after start of therapy Objective Responses* 3 wks 6 wks 9 wks 12 wks ______________________________________ Improved 4 2 2 -- Stable 17 11 8 6 Worse 13 21 24 28 ______________________________________ 5-Day course (27 patients) Time after start of therapy Objective Responses* 4 wks 8 wks 12 wks ______________________________________ Improved 1 -- -- Stable 12 7 6 Worse 14 20 21 ______________________________________ *3 patients showed 25%-50% response at 3 wks only.
Gottlieb and Luce reported the results of treatment of patients with malignant melanoma with camptothecin sodium (Cancer Chemotherapy Reports 56:103,1972). Fifteen patients with advanced malignant melanoma were treated with SCPT at doses of 90-360 mg/m.sup.2 repeated every 2 weeks. SCPT was administered as a single rapid intravenous (i.v.) injection starting at a dose of 120 mg/m.sup.2 repeated at 2-week intervals. The dose in subsequent courses was increased by increments of 60 mg/m.sup.2 per dose (to a maximum of 360 mg/m.sup.2) in eight patients who tolerated their initial doses with minimal toxicity. To prevent the known bladder toxicity of this drug, patients were well hydrated for 3 days after therapy. None of the patients had a 50% or greater decrease in tumor diameter. Less pronounced transient tumor regression was noted in three patients, but no clinical benefit was associated with these responses. The remaining patients had no change or progression in their disease. Toxic effects included myelosuppression (11 patients), nausea and vomiting (9 patients), alopecia (8 patients), diarrhea (3 patients), and hemorrhagic cystitis (1 patient). These investigators concluded that SCPT, at least as administered in this study, had little to offer the patient with advanced disseminated melanoma.
Creaven and co-investigators reported studies of plasma camptothecin levels during a 5 consecutive day course of treatment (Cancer Chemotherapy Reports 56:573-578, 1972). These investigators state that the toxicity of SCPT has been widely and unpredictably variable in the course of initial clinical evaluation. Severe toxic effects including cystitis occurred even though patients with obvious renal disease were excluded. In this study they investigated plasma camptothecin levels 24 hours after the administration of SCPT administered on a once daily.times.5 schedule to determine whether such measurements would be of value in predicting toxicity, and observed that plasma camptothecin levels have little relation to the dose given when the dose is in the range of 6.5-20 mg/m.sup.2 /day.
In another clinical study Muggia and co-workers reported the results of a Phase I Trial of weekly and daily treatment with SCPT (Cancer Chemotherapy Reports 56: 515-521, 1972). Fifteen patients were treated at four weekly dose levels ranging from 20 to 67 mg/m.sup.2 of SCPT. Reversible leukopenia was the major dose-limiting toxic effect. Five-day loading courses were begun at total doses of 1.5 mg/m.sup.2 per course because increased sensitivity to daily administration had been noted in animal studies. Leukopenia was more prolonged after daily treatment than after weekly treatment and occurred in four of six patients receiving a total dose of 100 mg/m.sup.2. Tolerance to 5-day courses was an unexpected clinical result. Also unpredicted by preclinical studies was human susceptibility to cystitis with either schedule of treatment. They noted clinical responses in two of ten patients in whom responses could be evaluated after weekly courses of treatment. No clinical benefit was observed in eight patients with measurable disease who were treated with the 5-day courses at dose levels associated with toxicity. Cystitis was another toxic effect occurring frequently after treatment with SCPT, and this toxicity was not predicted by preclinical toxicologic studies. The investigators suggested that the occurrence of cystitis may be related to the duration of the patient's exposure to the drug, and proposed that camptothecin is excreted unchanged by the kidneys, although a large percentage of the drug administered cannot be accounted for in the urine. They also proposed from this study that it is possible that relatively less drug is excreted in the urine of animals since an extremely active transport of camptothecin into bile had been demonstrated. They also postulated that the mucosa of the human bladder is more susceptible to the toxic action of camptothecin or that the effect on the human bladder is due to some unrecognized camptothecin metabolite.
There are several features which are common in these earlier clinical studies with SCPT. First is the use of SCPT ("SCPT") which made the camptothecin more water soluble. Hydrolysis of the lactone E-ring to form the water soluble carboxylate species was accomplished by formulating camptothecin in sodium hydroxide. The antitumor activity of the carboxylate form of camptothecin is reduced by at least 10-fold, which partially accounts for the lack of clinical response in these studies. Second is the rapid intravenous administration of the drug. Camptothecin is an S-phase specific drug and therefore will exert a greater antitumor effect under conditions of prolonged or repetitive exposure, as in a continuous intravenous infusion or repetitive daily dosing. The short infusion (i.v. push or rapid i.v. infusion) times in all of these earlier studies do not allow a long enough exposure time to the drug to attain suitable plasma drug levels, and is further compounded by the administration of the water soluble carboxylate form of camptothecin. A third common feature is the notable frequency of cystitis in these studies using water soluble SCPT.
The novel features of the present invention includes the following: (1) pharmaceutically acceptable formulations which allow direct parenteral administration of lactone stable highly lipophilic camptothecin derivatives to human patients with cancer (referred to as "HLCD"); (2) pharmaceutically acceptable formulations which allow the direct oral administration of lactone stable HLCD to human patients with cancer. The inventors predict that by administering the carboxylate species of HLCD a higher incidence of renal toxicity is likely to be observed than if the lactone species of HLCD is administered to patients.
The inventors maintain that the previous use of SCPT caused hemorrhagic cystitis relates to the enhanced renal excretion of the carboxylate form of camptothecin which when exposed to the lower pH (.about.pH 6 or less) of the distal convoluted tubule and collecting duct in the kidney, as significant proportions of the carboxylate form of camptothecin is converted into the lactone form. The formation of the lactone species in high concentration at the distal convoluted tubule and collecting duct resulted in a high concentration of the lactone form of camptothecin being excreted and damaging the uroepithelium which resulted in hemorrhagic cystitis. Elimination of a greater concentration of the lactone form of camptothecin by the renal route is enhanced by administration of the water soluble carboxylate form and is greatly reduced by administration of the lactone form of the drug. Thus, additional significant utilities of the present invention are that the administration of HLCD substantially in the lactone form orally or parenterally to cancer patients will significantly reduce the renal elimination of HLCD and further that the incidence of hemorrhagic cystitis will be significantly reduced in patients who receive the formulations of HLCD claimed in this invention.
In addition to the previously noted toxicities and limited clinical responses to camptothecin, HLCD have also been considered unsuitable for clinical use because they are all poorly soluble in water (e.g. less than 5 micrograms of HLCD per milliliter of water). The poor water solubility of HLCD requires the use of large volumes of water-based parenteral vehicle, which results in administering the drug for a prolonged period of time and causes inconvenience and discomfort to patients. Also, administering the drug for prolonged period of time increases the costs associated with treatment of patients. Hence, an HLCD formulation which permits higher concentrations of the active drug in the infusion after dilution with a suitable parenteral vehicle is desired. Also desired is that the drug remains in the diluted solution for sufficient amount of time to be effective. Such an infusion solution will be greatly beneficial to patients, by bringing down the time required for administering useful amounts of drug and also the costs associated with administering the drug for a more prolonged period of time.
One highly useful purpose of this invention is to formulate the HLCD in a pharmaceutically acceptable manner using an organic or a mixture of organic co-solvents with a high degree of physiologic safety to dissolve the HLCD in desirable concentrations and concurrently stabilize HLCD in the lactone E-ring form. It is this formulation invention which permits direct administration of HLCD to human patients with cancer.
The inventors believe that direct administration of a lactone stable HLCD to human patients has another important utility for treating patients with cancer. It is well known that cell membranes are comprised largely of lipid, and that lipid soluble drugs in general have superior ability to penetrate hydrophobic cellular membranes relative to water soluble drugs. HLCD are poorly water soluble and are therefore lipid soluble which facilitates their penetration into various body tissues and will improve the bioavailability and anticancer activity of the drug. The anticancer activity of the camptothecins in general is closely linked with their ability to inhibit the intracellular Topoisomerase I which is concentrated within the nucleus of the cell. The inventors contend that the present invention increases the amount of HLCD drug diffusion through the cellular and nuclear membranes in tumor cells and will result in superior antitumor activity of the drug. Water soluble camptothecin derivatives are predicted to have a lesser ability to diffuse through the cellular and nuclear membranes in the body.
The utility of suitable organic solvents for this invention involving pharmaceutical dosage forms of HLCD is restricted to those which have a high degree of physiological safety in humans. This invention teaches new methods for making pharmaceutical formulations for a variety of lactone stable HLCD with water solubility of 5 micrograms per milliliter or less. Some examples of poorly water soluble camptothecins (HLCD) include 10,11-methylenedioxy camptothecin, 10,11-ethylenedioxy camptothecin, 7-ethyl camptothecin(SN22), 7-ethyl-10-hydroxy camptothecin (SN38) and congeners thereof. Any poorly water soluble camptothecin with a solubility of 5 micrograms per milliliter of water or less may be dissolved or suspended in these novel formulations and will have appreciable quantities (greater than 90%) of the lactone form of drug in the resulting solution. 10,11-Methylenedioxy camptothecin, 10,11-ethylenedioxy-camptothecin, 7-ethyl camptothecin, and 7-ethyl-10-hydroxy camptothecin and congeners thereof are reportedly very active in preclinical studies, but they are also reported to be poorly soluble in water (less than 5 micrograms of drug will dissolve in one milliliter of water) which limits their utility because of the inability to readily administer these drugs to human patients with cancer (Pommier, et al. 1992, Wall et. al. 1994).
One of the advantages of the instant invention is that the instant formulations provide clinicians with the ability to directly adjust the plasma levels of HLCD to the point of therapeutic tolerance by controlling the dose and the schedule of drug administration. The inventors contend that this should lead to a superior ability to achieve more effective antitumor activity and reduced interpatient variability of the plasma levels of HLCD.
The different observations made in these studies suggest that direct administration of HLCD by parenteral and oral administration could provide significant clinical benefit for patients undergoing treatment for cancer. However, in the past, HLCD have been considered insufficiently water soluble for clinical use. The current invention overcomes the solubility problem by providing lactone stable pharmaceutically acceptable formulations of HLCD which upon dilution with an acceptable parenteral vehicle gives a stable solution of useful concentrations of HLCD for parenteral use and also a concentrated solution or suspension of HLCD suitable for encapsulation within a gelatin capsule for oral HLCD formulations.